Make a given layout display without borders. This is useful for
full-screen or tabbed layouts, where you don't really want to waste a
couple of pixels of real estate just to inform yourself that the visible
window has focus.

In order of increasing ambiguity (less borders more frequently), where
subsequent constructors add additional cases where borders are not drawn
than their predecessors. These behaviors make most sense with with multiple
screens: for single screens, Never or smartBorders makes more sense.